This invention relates to a cantilever for use with an atomic force microscope and a process for producing the same. More particularly, the invention relates to cantilevers equipped with styli that are indispensable for use in scanning atomic force microscopes (AFMs) which are becoming increasingly popular as tools for profiling surface geometries at atomic resolutions. The invention also relates to a process for producing such cantilevers. Specifically, the invention relates to cantilevers that are effective for use not only with AFMs but especially with scanning Maxwell stress microscopes (SMMs), as well as a process for producing such cantilevers.
Heretofore marketed or disclosed cantilevers for use with atomic force microscopes of the type contemplated by the present invention are classified into three major types as illustrated in FIGS. 6a, 6b, and 6c. The cantilever shown in FIG. 6a is currently available on the market and is characterized by comprising a single-crystal silicon base 51 which is processed to a cantilever beam 52 and a stylus 53 formed primarily by wet etching with an aqueous KOH solution. The angle of the tip of the stylus 53 is determined by the face direction of the single-crystal silicon base 51 and is typically about 70 degrees since a commonly employed base has a (100) face.
FIG. 6b shows the cantilever published by T. R. Albrecht et al., in 1990. Base 51 is made from a (100) single-crystal silicon film and has a cantilever beam 52 and a stylus 53 which are formed of either a thermally oxidized film or a nitride film (T. R. Albrecht et al., J. Vac. Sci. Technol., A8 (1990) 3386-3396). With this cantilever, it is extremely difficult to produce an adequately sharp point at the tip of the stylus 53 made from the thermally oxidized film and the stylus tip of the cantilever is not sufficiently sharp.
The cantilever shown in FIG. 6c was published by M. M. Farooqui in 1992 and it comprises an n-type silicon base 51 having a highly boron doped layer formed on a surface from which a cantilever beam 52 and a stylus 53 were formed (M. M. Farooqui et al.; Nanotechnology, 3 (1992) 91-97). With this cantilever, the thickness of the boron diffusion layer is equal to the sum of the height of the stylus 53 and the thickness the cantilever beam 52 and the portion of that layer which remains after processing the stylus 53 by dry etching provides the thickness of the cantilever beam 52. Therefore, the thickness of the stylus 53 and the thickness of the cantilever beam 52 are highly dependent on the conditions for dry etching the stylus 53.
FIG. 6d shows a cantilever published by L. C. Kong et al., in 1993. A single-crystal silicon base 51 is furnished with a cantilever beam 52 that is formed from a silicon oxide film or a nitride film and the beam 52 is overlaid with a polysilicon layer, which in turn is
processed by dry etching and thermal oxidation to form a stylus 53 (L. C. Kong et al. J. Vac. Sci Technol. B11 (1993) 634-641). Since this cantilever has small (ca. 0.1 .mu.m) grain boundaries in the polysilicon layer, it is difficult to sharpen the tip of the stylus to less than 0.1 .mu.m and, as a matter of fact, the published cantilever has a stylus tip of about 0.1 .mu.m.
Cantilevers suitable for use with AFMs must satisfy the following conditions:
(1) stylus 53 not only has a sufficiently sharp tip to permit profile mapping at atomic resolutions but also has been processed to provide a large aspect ratio (the ratio of stylus height to the diameter of its bottom); PA1 (2) cantilever beam 52 has an invariable spring constant; and PA1 (3) cantilever beam 52 is resistant to mechanical damage during handling. PA1 (a) a substrate composed of a (100) single-crystal silicon base having adequate mechanical strength, a silicon oxide film formed thereon by thermal oxidation, and a (100) single-crystal silicon film formed on said thermally oxidized film by bonding is oxidized thermally to form a thermally oxidized film 0.5-1 .mu.m thick on the surface of the single-crystal silicon film over said thermally oxidized silicon film; PA1 (b) the thermally oxidized film on the surface of said bonded single-crystal silicon film is processed to a circle with a diameter of 10-15 .mu.m; PA1 (c) with said circularly processed thermally oxidized film being used as a mask, said bonded single-crystal silicon film is processed to a generally conical stylus geometry by the combination of reactive ion etching (RIE) and wet etching with an aqueous KOH solution and, at the same time, the thermally oxidized film which underlies said bonded single-crystal silicon film is exposed; PA1 (d) said exposed thermally oxidized film is processed to a cantilever beam geometry; PA1 (e) a thermally oxidized film is formed on all surfaces of the structure formed by steps (a) to (d); PA1 (f) a resist is applied to a thickness of at least 5 .mu.m so as to protect said roughly processed conical stylus and the resist is then processed in registry with the cantilever beam geometry as processed in said step (d); PA1 (g) the thin thermally oxidized film which has been formed as the resist in step (e) is processed to the cantilever beam geometry so as to have the underlying silicon base exposed and said resist is then stripped; PA1 (h) the silicon base around the cantilever beam geometry which has become exposed in said step (g) is etched away by wet etching with an aqueous KOH solution so that a cantilever beam formed of said thermally oxidized film is freely suspended from said silicon base; PA1 (i) the thin thermally oxidized film which has been formed on the surface of the stylus in said step (e) is etched to provide a sharp tip; PA1 (j) the base from which the cantilever beam has become freely suspended in said step (h) is cut off at the site lying underneath said cantilever beam; and PA1 (k) all surfaces of the structure that have been formed in said steps (a)-(j) are covered with a thin electroconductive film.
To meet the second requirement, the cantilever beam has to be formed with high reproducibility and consistency in terms of not only the quality of the film from which it is made but also its size, especially its thickness. None of the prior art cantilevers shown in FIG. 6 satisfy all of the conditions (1)-(3). With the cantilevers shown in FIGS. 6a, 6b and 6d, it is theoretically difficult to sharpen the tip of stylus 53. The cantilever shown in FIG. 6c has an inherent defect in that the height of stylus 53 and the thickness of beam 52 are highly sensitive to variations in the processing conditions for their formations. At the same time, the cantilever beam which is formed of the highly concentrated boron diffusion layer (10.sup.20 B atoms/cm.sup.3) with great internal stress has inherent flex and its spring constant will change non-linearly under deflection, which presents a serious obstacle to measurements with AFM.
A common problem with the four prior art cantilevers is that the height of stylus 53 is no more than about 3-5 .mu.m and the aspect ratio 1 or less. In addition, the cantilever beam 52 in all prior art cases is completely exposed from the base 51 and has no protecting mechanism of any kind.